Telemetry unit

ABSTRACT

A telemetry unit ( 100 ) is provided for mounting to the inside of a pneumatic tire, which includes sensors for measuring the pressure and temperature within the tire and an RF transmitter for transmitting measured data to an on-board display remote location. The unit ( 100 ) includes a piezoelectric element ( 114 ) supported in a housing ( 112 ) with an actuator ( 136 ) arranged for contact with the element ( 114 ), to deflect the element ( 114 ) in response to external forces acting on the actuator ( 136 ) during rotation of the tire. For every rotation of the tire, cyclic pulses of electrical charge are generated by the deflection of the element ( 114 ). The unit ( 100 ) includes control means which controls the consumption of stored charge when measuring and transmitting data and is arranged to vary the rate of transmission of data from the unit ( 100 ) in dependence on the rotary speed of the tire.

BACKGROUND OF THE INVENTION

The present invention relates to a telemetry unit for a tyre monitoringapparatus.

It is known to provide a tyre monitoring apparatus for measuring thepressure within vehicle tyres. The tyre monitoring apparatus may alsomeasure other parameters within a tyre environment, such as the localtemperature of a tyre. The measured data is transmitted, for example viaa radio wave link, to the cabin of the vehicle where it iselectronically processed before being displayed to the vehicle driver.This enables the recipient of the transmitted data to monitor changes inthe condition of the tyre, for example to reduce damage to the tyre(s)of a vehicle, or to predict tyre failure. This is of particularadvantage at high vehicle speeds, when the environment within a tyre isat its most hostile and the likelihood of damage to a tyre and, indeed,injury to the occupants of the vehicle, is at its greatest.

The majority of existing tyre monitoring apparatus use a battery as thepower source, which is located on or within a wheel or tyre. Sucharrangements have several undesirable limitations, for example limitedbattery life and size or weight which can be accommodated within a tyre.This can have a further undesirable knock on effect, in that if there isa limited power source available, for example as a result of weightimplications, the number and frequency of data transmissions that can berelayed for processing is compromised.

It is an object of the invention to reduce or substantially obviate thedisadvantages referred to above.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, there is provided a telemetry unitfor a tyre monitoring apparatus, the telemetry unit including a housinghaving sensor means for measuring data relevant to the local environmentof the telemetry unit, transmission means for transmitting measured datato a remote location, a piezoelectric power generator adapted togenerate and store an electrical charge in response to rotation of thetyre for powering the unit, and control means for controlling the sensormeans and transmission means, in which the control means is arranged tovary the rate of transmission of data from the telemetry unit independance on the rotary speed of the tyre.

Preferably, the piezoelectric power generator is arranged to produce atleast one pulse of electric charge for each rotation of the tyre, andthe control means is adapted to monitor the number and/or frequency ofthe pulses generated in order to determine the appropriate rate oftransmission of data.

The control means is adapted to initiate monitoring of the localenvironment and transmission of measured data after a predeterminednumber of pulses has been detected.

In a preferred embodiment, the housing is adapted to be mounted to aninner wall of a tyre.

An exterior surface of the housing may have a substantially arcuateprofile adapted for bonding to an arcuate interior surface of a vehicletyre.

An exterior surface of the housing may include an external profile forcomplimentary engagement with the internal pattern of a vehicle tyre.

In a preferred embodiment, the housing is releasably mounted on afooting adapted to be bonded to the internal wall of a tyre, for exampleby means of clips. The footing may include air channels for allowingmovement of air about the housing, in use.

Preferably, the sensor means includes at least one of a pressure sensorand/or a temperature sensor.

Conveniently, an actuator is movably mounted in the housing and adaptedfor contact with the piezoelectric element under centrifugal forces.

The invention is advantageous in that it provides a telemetry unit whichis suitable for measuring and transmitting data from inside the harshenvironment of a rotating pneumatic tyre. Measurement and transmissionof data from within the tyre is possible for normal operating conditionswithin a tyre, for detecting potential failure or blow out of a tyre,for example.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is an exploded perspective view of an in-tyre power/sensor ortelemetry unit in accordance with a preferred embodiment of theinvention;

FIG. 2 is a partial cross-sectional view of the unit shown in FIG. 1 inan assembled, rest position;

FIG. 3 is a perspective view the unit shown in FIG. 2;

FIG. 4 is a schematic plan view of the piezoelectric disc and brassmounting which forms part of the unit shown in FIGS. 1 to 3;

FIG. 5 a is a block diagram showing the interrelationship betweencomponents of the power generator;

FIG. 5 is a flow diagram showing the stages involved in a low powerconsumption protocol for controlling the measurement and transmission ofdata from the unit shown in FIGS. 1 to 3;

FIG. 6 is a perspective view of a further embodiment of a telemetry unitin accordance with a further preferred embodiment of the invention;

FIG. 7 is perspective exploded view of the unit of FIG. 6, from above;

FIG. 8 is a perspective exploded view of the unit of FIGS. 6 and 7, frombelow;

FIG. 9 is a cross-sectional view through the unit of FIGS. 6 to 8;

FIG. 10 shows an end view of the unit of FIGS. 6 to 9 in use in apneumatic tyre; and

FIG. 11 is a side view of the unit as shown in FIG. 10.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1 to 4, a power generator/sensor unit is indicatedgenerally at 10, for use in a tyre monitoring apparatus. The unit 10includes a housing 12 made as a reinforced injection moulding compositefor mounting in and adapted to withstand the harsh environment ofpneumatic vehicle tyre. Although the housing 102 is described as beingmade as a composite moulding, any suitable material can be used.

The housing 12 has a base or footing 16 having a shallow convex outerprofile, indicated at C in FIG. 2, for bonding to a correspondinglyarcuate interior surface of a vehicle tyre. The base 16 defines achamber, indicated at 18 in FIG. 1, having an internal base wall 20.

The unit 10 includes a piezoelectric element 11 in the form of apiezoceramic disc 14 having a radius R, which is mounted centrally on abrass supporting disc 15 having a radius greater than R. The element 11is mounted in the housing 12 for generating electrical power to operatecircuitry within the unit 10.

The base 16 of the housing 12 includes two opposed recesses 22, one ofwhich can be seen clearly in FIG. 1, for supporting part of theperiphery of the brass disc 15. When supported on the base 16, thecentral portion of the brass disc 15 is spaced apart from the base wall20 by a small distance. A cover 26 is received on the base 16, whichoverlays the part of the periphery of the brass disc 15 supported on therecesses 22, such that the disc is clamped between the cover 26 and therecesses 22 along two edge portions 47.

A cap 28 is provided over the cover 26, the cap including a centralformation 30 which extends through a central aperture 27 in the cover26.

A printed circuit board (PCB) 32 is mounted in the housing 12 on the cap28. As shown in FIG. 5 a, the PCB 32 includes a micro processor, a radiofrequency (RF) transmitter, pressure and temperature sensor circuitry,including pressure and temperature sensors, and supervision and controlcircuitry, which form part of a tyre monitoring apparatus. The PCB 32also includes a rectifier for converting an alternating current outputfrom the piezoceramic disc 14 into a direct current output; an energystorage element in the form of a series of a capacitors, which store thedirect current output from the rectifier until required, and a DC-DCcontroller which is provided for regulating voltage output from thecapacitors. The unit 10 uses ultra low leakage type capacitors, toensure that as high a percentage of the generated charge is retained aspossible and that internal leakage is kept to a minimum.

The PCB 32 is in electrical communication with the piezoceramic disc 14via two wires, not shown, and is securably located on the cap 28 by apotting compound 34, to protect the PCB 32 during installation ortransit, and from the harsh environment within a rotating pneumatictyre. The potting compound 34 can be any suitable type but in thisembodiment is a two-part epoxy adhesive.

An actuator 36 is disposed between the piezoceramic disc 14, the cover26 and the cap 28, the actuator consisting of an integrally formed foot38 and a stem 40. The stem 40 extends into the central formation of thecap 28 and includes a central bore 42. As can be seen clearly in FIG. 2,the foot 38 includes an integrally formed elongate projection or nose44, which is in contact with the piezoceramic element. The nose 44extends diametrically across the piezoelectric element 11, as indicatedin FIG. 4, which shows the contact area 45 of the nose 44 on thepiezoelectric element 11 and the areas of support 47 for the disc 15 onthe base 16. It will be appreciated that the piezoelectric element 11 isconfigured substantially as a simply supported beam, supported on oneside by the recesses 22 in the base 16 and contactable on its oppositeside by the nose 44 of the actuator 36.

The actuator 36 is connected to the cap 28 by a screw 46 which passesthrough the cap 28 and is securably received in the bore 42 of the stem40. The base 16 is connected to the cover 26 by four screws 48, whichpass through the corners of the base 16 and which are securably receivedin the cover 26.

The arrangement is such that the piezoelectric element 11 can bedeflected downwardly (as viewed in FIG. 2) under the influence of theactuator 36, as will be described in more detail below. However, themaximum deflection of the piezoelectric element 11 is limited by thedistance between the underside of the brass disc 15 and the internalbase wall 20, set at 0.4 mm in the embodiment of FIGS. 1 to 4. Thus, theelement 11 is protected against excess deflexion, which might otherwisedamage the structure and generating capacity of the element 11. Themovement of the actuator 36 within the housing 12 in the oppositedirection, i.e. perpendicularly away from the piezoceramic disc 14,upwards as viewed in FIG. 2, is restricted by walls 27 of the cover 26.In the embodiment of FIGS. 1 to 4, the maximum distance between theupper side of the foot 38 of the actuator 36 and the walls 27 of thecover 26 is 0.6 mm when the power generator 10 is in the rest positionshown in FIG. 2. Hence, the maximum travel of the actuator 36 within thehousing 12 is 1 mm in the embodiment of FIGS. 1 to 4. This maximumdistance of travel of the actuator 36 within the housing 12 is set at apredetermined low value to protect the piezoceramic disc 14 from damagedue to deflection and/or impact of the actuator 36 on the upper surfaceof the piezoceramic disc 14 in use. It will be understood that themaximum travel of the actuator and deflection of the piezoelectricelement can be limited to any distance suitable for protecting theintegrity of the structure and charge generating capacity of thepiezoelectric element.

The arrangement of the piezoceramic disc 14, in combination with thecomponents of the PCB 32 which are associated with the piezoceramic disc14, as described above, form part of a power generator, for supplyingpower for the circuitry of the unit 10.

Operation of the power generator will now be described, by way ofexample, in which the unit 10 is mounted in a pneumatic tyre on thewheel of a vehicle, with the outer surface of the base 16 of the housing12 bonded to a correspondingly arcuate profile of an interior surface ofthe tyre, and in which the unit 10 includes a piezoceramic disc 14 ofany suitable known construction.

It will be appreciated that mechanical excitation of the disc 14generates a voltage. The effect is substantially linear, i.e. theelectric field generated varies directly with the applied mechanicalstress, and is direction dependent, so that compressive and tensilestresses generate voltages of opposite polarity.

The cap 28, PCB 32, potting compound 34 and the actuator 36 act on thedisc 14 as a single unit mass, in use, i.e. with the cap, actuator,circuitry and potting compound acting as a composite actuating mass.When the wheel is in rotation, centrifugal forces act on the cap 28, PCB32 and the potting compound 34, which urge the actuator 36 radiallyoutwards in the direction of the piezoelectric element 11. Thiscentrifugal action on the actuator 36 causes the piezoelectric element11 to deflect, typically between 0.2 to 0.4 mm at its central region 45from a rest position when the wheel is not in rotation. Since thepiezoelectric element 11 acts as a simply supported beam and the nose 44of the actuator 36 is in contact with the disc 14 at the centralposition 45 between the area of support for the brass disc 15, thedeflection is in the form of a uniform bending of the discs 14 and 15between the two areas of support 47 of the brass disc 15.

It will be understood that, as the vehicle is in motion, the externalarea of the tyre adjacent the unit 10 comes in to contact with thesurface along which the vehicle is travelling, once with everyrevolution of the wheel. This contact deforms the area of the tyreadjacent the unit, which deformation is transmitted to the powergenerator, ultimately in the form of a deformation of the piezoelectricelement 11 by the actuator 36. Hence, the piezoceramic disc 14 issubjected to variations in mechanical excitation during rotation of thewheel on the road surface, whereby each excitation results in apotential difference being generated by the piezoceramic disc 14. Thisprocess is set out below, with reference to a rotating wheel startingfrom a position where the area of the tyre adjacent the unit 10 ismoving towards contact with a road surface.

With the wheel in rotation, the actuator 36 is in contact with thepiezoceramic disc 14, under centrifugal action from the cap 28, PCB 32and potting compound 34, as described above. The piezoceramic disc 14therefore experiences a substantially constant deflection under thecentrifugal forces which are transmitted through the actuator 36. As thewheel rotates further, the area of the tyre adjacent the unit 10 comesinto contact with the road surface and deforms. The deformation resultsin a deceleration of the tyre in the region of the point of contact withthe road surface, causing a sudden reduction in the centrifugal forcesexperienced by the actuator 36, almost instantaneously, substantially tozero. This change in centrifugal acceleration causes a reduction in thedeflection experienced by the piezoceramic disc 14 under action of theactuator 36 and generates a first pulse of electrical charge, which iscommunicated to the PCB 32.

As the wheel rotates further, at the instant where the area of the tyreadjacent the unit 10 moves away from contact with the road surface, theacceleration of the tyre adjacent the unit 10 increases suddenly, whichresults in an instantaneous increase in the centrifugal forcesexperienced by the actuator 36. Hence, piezoceramic disc 14 is againcaused to deflect under centrifugal action of the actuator 36, cap 28,PCB 32 and potting compound 34, as described above, which generates asecond pulse of electrical charge of opposite polarity to the firstpulse described above, which is communicated to the PCB 32.

Hence, during a single revolution of the wheel two pulses of electricalcharge, of opposite polarity, are generated in quick succession,constituting a single alternating current output. The rectifierrectifies the alternating current output into a direct current output,which is stored in the capacitors for use to power the tyre monitoringapparatus. For each revolution of the wheel, a small storable electricalcharge is generated, typically of 5-10 nano coulombs.

In addition to the storable charge generated with each revolution of thewheel due to contact with the road surface, the unit 10 may alsotransmit other excitation forces to the piezoelectric element 14, forexample accelerations/deflections which are caused by vibrations due toimperfections in the road surface, or out of balance forces on the wheelitself. If the excitation is sufficient to cause deflection of thepiezoelectric disc 14, an additional storable charge will be generatedand stored in the capacitors, as described above.

In some circumstances, the forces acting on the unit 10 inside a vehicletyre will not be sufficient to cause uniform bending of the piezoceramicdisc 14, as described above. Instead, the deformation will be in theform of a localised ‘squashing’ of the structure of the disc 14 at thepoint of contact with, and in the region immediately adjacent to, theactuator. In operation, the localised ‘squashing’ of the disc structurealso generates a potential difference across the element 11, forgenerating charge substantially as described above.

The unit 10 is particularly advantageous in that the control circuitryis used as an actuating mass for the piezoelectric element 11. In thedescribed embodiment, the weight of the cap 28, the PCB 32 and thepotting compound 34 operate as a single unit to act as an actuatingmass/exciter for the piezoceramic disc 14, without the need for anyadditional mass. Hence there is an overall saving in weight in the powergenerator, to minimise localised wear caused by the unit 10 adjacent thearea of mounting in the vehicle tyre, and thus reduce the likelihood ofa localised bald spot occurring in the tread of the tyre.

The outer surface C of the base 16 may include an external profile forcomplimentary engagement with the internal pattern of a vehicle tyre, tolimit further the effects of localised wear on the tyre, in use.

In order to utilise the small amounts of power generated by the powergenerator and to remove the need for a battery backup to power the tyremonitoring apparatus, an ultra low power consumption protocol is used tocontrol the consumption of power stored by the capacitors.

Operation of a tyre monitoring apparatus will now be described by way ofexample, illustrating the stages which are implemented to ensure thatthe optimum low power protocol is realised, starting with the monitoringapparatus in a ‘sleep’ mode, with reference to FIG. 5. As referred toabove, the tyre monitoring apparatus includes a unit 10 having apiezoelectric power generator, a micro processor, a radio frequency (RF)transmitter, pressure and temperature sensor circuitry and supervisionand control circuitry.

EXAMPLE 1

Stage 1

The micro processor is in ‘sleep’ mode, in which all internal processingis suspended, apart from a monitor circuit, for monitoring the ‘wake up’requirements of the micro processor. In this embodiment, the monitorcircuit monitors an externally referenced clock in the form of a crystaloscillator, located outside the micro processor in the power/sensorunit. Hence, in sleep mode, the majority of the micro processorcircuitry is disabled and the power consumption of the tyre monitoringapparatus is at minimum level, for example approximately 24 micro ampereof supply current.

Stage 2

After a predetermined time, in this embodiment 60 seconds, the monitorcircuit ‘wakes up’ the micro processor. Upon ‘wake up’, the microprocessor switches from the external clock to an internal clock, in theform of an internal resistor capacitor oscillator. This switch isimplemented to facilitate a higher speed operation of the analogue todigital conversions and subsequent calculations which are utilised bythe tyre monitoring apparatus. The switch also initiates power to theinternal circuitry of the micro processor, which allows the main programof the micro processor to be used and to enable the micro processor toenter a measure and control phase.

Stage 3

Once the micro processor has ‘woken up’, power is provided to thetemperature and pressure sensor circuitry. A prescribed time is thenallowed to elapse, in this embodiment 0.5 milliseconds, to facilitatesettling of the sensor circuitry, after which time the micro processormeasures the local pressure and temperature within the tyre. The valuesare then stored within the micro processor and the power to the sensorcircuitry is removed instantaneously.

Stage 4

The stored pressure and temperature values are concatenated with asensor identification and cyclic redundancy check to form a data packetfor transmitting to a receiver unit/display unit in the vehicle.

Stage 5

The micro processor then switches from the internal clock back to theexternal clock. This change is employed to ensure accurate time signalsfor the transmission of the data via the radio frequency (RF) link,since the external clock is a quart crystal time reference unit, whichensures that a higher absolute frequency accuracy is attainable thanwith the internal clock.

Stage 6

The micro processor sets a control line to a logic high of 3v, whichenables the RF transmitter, thus causing it to emit a radio frequencycarrier. A settling time of approximately 1 millisecond then elapses tofacilitate settling of the RF transmitter components prior to thetransmission of data from the PCB 32. A pseudo bit pattern, used to biasa radio frequency data slicer, is then concatenated with the sensoridentification and cyclic redundancy check for transmitting. The data tobe transmitted is then frequency modulated onto a 433 MHz radio wave forpropagation to the receiver unit.

Stage 7

The data is transmitted and power to the RF transmitter is theninhibited instantaneously, at which point the micro processor thenre-enters ‘sleep mode’.

Hence, by utilising the low power protocol described in stages 1-7 ofthe above example, the tyre monitoring apparatus utilises only a minimumamount of power from the power generator, to transmit a reading of thelocal pressure and temperature within the tyre. After use, the microprocessor remains in sleep mode for a predetermined period, as referredto in Stage 2 above, while the energy stored in the capacitors isrecharged by excitation of the piezoceramic disc 14, as described withreference to FIGS. 1 to 4. Hence, using a continuous cycle of stages1-7, the tyre monitoring apparatus is able to monitor the localcondition of the tyre, utilising the small electrical charges generatedby the piezoceramic disc 14, without the need for a back-up batterysupply. The continuous cycles are of advantage during normal operatingconditions of the tyre, whereby any changes in tyre pressure ortemperature, which might indicate a potential problem or failure of thetyre can be monitored, to avoid a blow out, for example. This hasparticular advantage at high vehicle speeds.

Principally, there is a tri-way interdependency of critical factors inthe protocol for the telemetry unit, between the charge generationcapability of the piezoelectric element, the charge storage size andefficiency, and the RF transmitter reliability governed by thetransmitter ‘on’ time. For a given type of piezoelectric element, thereis an optimum charge capacitance for the power generator and optimumtransmission time for the RF transmitter. The piezoelectric element musthave sufficient charge generation overcome the impedance of the storagecapacitors, and the capacitors must have sufficient capacitance to holdthe charge required to perform the measurement/transmission cycle. TheRF transmitter ‘on’ time, i.e. when the transmitter is active andtransmitting, must be optimised between a maximum period in which thereis sufficient charge to transmit the data prior to the energy storagebeing exhausted, and a minimum period below which the reliability of theRF link is adversely effected. If transmission time is extended beyondthe optimum period, the effective frequency of data transmissions isreduced for a given capacitance.

The data transmitted to the in-car receiver unit is shown to the driverof the vehicle on the display unit for the or each of the sensorcircuits in the tyre monitoring apparatus, with respect to each tyre ofthe vehicle. The display unit informs the driver of the data visuallyand/or by audible means, for example a link to the audio system in thevehicle.

Each tyre/wheel of the vehicle is marked by an individual identifyingfeature that relates to a specific sensor located within that tyre. Thisidentifying feature is also represented on the display unit, incombination with the data from the sensor within the tyre. In the eventthat the wheel is moved to another position on the vehicle it can alwaysbe related to the relevant information on the display unit. Suitableidentifying features include colour-coded symbols and alpha numericsymbols.

Each sensor has a unique electronic serial number, which can be used toaid the security of the radio transmission data. The unique electronicserial number can also act as an electronic tagging feature for securityand anti counterfeiting purposes.

With reference to the preferred embodiment of the power generator, ithas been described that a storable electrical charge is generated by thepiezoelectric element with each revolution of the vehicle wheel.Therefore, it will be appreciated that the generation of charge isproportional to the speed at which the vehicle is travelling. In theabove example of the power consumption protocol, the time delay betweentransmission of data from the tyre monitoring apparatus and the “wakeup” of the micro processor for measuring and transmitting a furtherreading is set to a predetermined value. In a slow moving vehicle, theelectrical charge which is generated and stored within a predeterminedtime period is less than would be generated and stored in a vehicletravelling at a faster speed in the same time period. Therefore, thetime interval between “wake up” of the microprocessor is set at apredetermined value, selected to allow a sufficient electrical charge tobe generated and stored for measurement and transmission of theparameters of a tyre on a slow moving vehicle, for example 25 kmh.

However, as the speed of the vehicle increases, the rate of electricalcharge generation also increases. Thus, the time period required togenerate sufficient electrical charge to enable the tyre monitoringsystem to measure and transmit the tyre parameters is reduced.

To take advantage of this, the low power protocol described above can bemodified so that the micro processor is “awoken” from its sleep mode atintervals relative to a function of the speed of the vehicle or thestate of the electrical charge stored in the capacitors, which enablesthe transmission of data to be varied in proportion to the speed of thevehicle.

The following example shows a preferred mode of operation, in which therate of transmission of data from the tyre monitoring apparatus isproportional to the speed of the vehicle, starting with the monitoringsystem in a “sleep” mode, substantially as described in example 1.

EXAMPLE 2

Stage 1

As the wheel rotates, storable power outputs are produced by the powergenerator, one per revolution, as described above. In this example, thischaracteristic of the power generator is used to monitor the speed ofthe vehicle and/or the state of charge of the capacitors. A smallportion of each storable power output is signal conditioned to take into consideration false triggers of power which may be experienced by thepiezoelectric disc 14 during rotation of the wheel, for exampleaccelerations/deflections which are caused by vibrations due toimperfections in the road surface. The conditioned signal is thensupplied to an interrupt circuit in the micro processor, whichmomentarily wakes the micro processor from its sleep mode and incrementsa counter in the micro processor. The micro processor then returnsinstantly to the sleep mode.

Stage 2

Both the average charge generated per revolution of the wheel and thevalue of stored charge sufficient to measure and transmit data from theunit 10 are known. Hence, the number of “interrupts” or increments ofthe counter required for the capacitors to store a charge sufficient formeasurement and transmission of data from the apparatus can becalculated. Therefore, the micro processor can be set to “wake up”,substantially as described in stage 2 of example 1, after apredetermined number of revolutions of the wheel, for example 50revolutions. At this point, power is initiated to the internal circuitryof the micro processor, which allows the main program of the microprocessor to be used and to enable the micro processor to enter ameasure and control phase.

The internal clock of the micro processor monitors the time taken forthe predetermined number of revolutions to be completed. Hence, a valueof average speed of the vehicle during the time period can be calculatedfrom the elapsed time and the distance travelled which iscross-referenced from a table of data relating to the diameter of thewheel.

Stage 3

As described in example 1, once the micro processor has ‘woken up’,power is provided to the temperature and pressure sensor circuitry. Aprescribed time is then allowed to elapse, for example 500 microseconds, to facilitate settling of the sensor circuitry, after whichtime the micro processor measures the local pressure and temperaturewithin the tyre. The values are then stored within the micro processorand the power to the sensor circuitry is removed instantaneously.

Stage 4

The stored pressure and temperature values are concatenated with asensor identification and cyclic redundancy check, as described in stage4 of example 1, and the value of speed calculated during stage 2.

Further stages 5 to 7 are then carried out substantially as describedwith reference to stages 5 to 7 in the above example.

Since the speed of the data transmissions is proportional to the speedof the vehicle, this mode of operation provides a major safetyimprovement over known tyre monitoring apparatus, in that theinformation is transmitted and updated regularly, depending on the speedof the vehicle. This has particular advantage in that a catastrophicfailure of a tyre is more likely to occur, possibly with greaterconsequences, at high vehicle speed. The unit 10 is more regularlyupdated at high vehicle speeds than at lower speed, thereby improvingvehicle safety by warning the driver of any deflation of the vehicletyres, for example.

A power/sensor unit according to a further embodiment of the inventionis indicated at 100 in FIG. 6 to 11, which corresponds substantially tothe unit 10 described above.

As shown in FIG. 6, the unit 100 includes a housing 112, which consistsof a base portion 116 and a cap 128 mounted on the base portion 116. Thehousing 112 is removably mounted on a resilient base or footing 151 madeof a rubber or any other suitable material. A pair of resilient cliparms 153 are pivotably provided on the footing 151, for snap-fittingengagement with formations 117 on the base portion 116 of the housing112. The unit 100 can be simply removed from the footing 151 byunclipping the arms 153 from their engagement with the formations 117,for repair or installation in another tyre using a new footing 151, forexample.

The footing 151 is adapted to be permanently secured to an internalsurface 159 of a tyre, as shown in FIGS. 10 and 11, and can be disposedof with the tyre after use. Two air channels 155 are provided in thefooting 151, which have the dual function of allowing air movement aboutthe unit 100, in use, and providing a footing of sufficient flexibilityto aid protection and shock absorption for the internal components ofthe unit 100, whilst propagating the flexure of the tyre during rotationto the internal components of the unit 100.

The footing 151 is generally elliptical and has a greater surface areathan the base portion 116 of the housing 112. The shape and size of thefooting 151 is designed to spread the load of the unit 100 on a tyre, toreduce adverse tyre wear in the region of the unit 100, that mayotherwise be expected when providing a localised mass on the inside of atyre, the mass of the unit 100 being in the region of between 30-50grams.

Referring specifically to FIGS. 7 to 9, the internal configuration ofthe housing 112 and the internal components of the unit 100 will now bedescribed.

The unit 100 includes a piezoelectric element 114 mounted on a brasssupporting disc 115, substantially as described with reference to FIGS.1 to 4. The base portion 116 of the housing 112 defines a compartment118 formed by a base wall 120 and a peripheral wall 121. Recesses 122are formed in the peripheral wall 121, for supporting a part of theperiphery of the brass disc 115. When supported on the base portion 116,the central portion of the brass disc 115 is spaced apart from the basewall 120. In this embodiment, tabs 123 are provided which extend over aportion of the recesses 122, for engagement with the periphery of thebrass disc 115, for retaining the brass disc 115, and thereby thepiezoelectric element 114, on the base portion 116.

The unit 100 includes a one-piece moulded actuator 136 defining achamber 137, which is movably mounted in the housing 112. A printedcircuit board or PCB (not shown), corresponding to the PCB 32 describedwith reference to the embodiment of FIGS. 1 to 4 is mounted in thechamber 137. The PCB is in electrical communication with thepiezoceramic disc 114 via wires (not shown), which pass through anaperture 139 in the floor of the chamber 137. The PCB is securelylocated on the actuator 136 by a potting compound (not shown), whichprotects the PCB during installation or transit of the unit 100, as wellas from the harsh environment within a rotating pneumatic tyre in use.

An elongate projection or nose 144 is formed on the underside of theactuator 136, as can be seen in FIG. 8. In a normal rest position in thehousing 112, the nose 144 is in contact with the piezoceramic disc 114,as can be seen in FIG. 9. In the rest position, the underside of theactuator 136 is spaced from an internal surface 141 of the base portion116 by a distance of approximately 0.3 mm.

In use, the piezoelectric element 114 is deflected in the direction ofthe base wall 120 under action of the actuating mass, and it will beappreciated, therefore, that the maximum deflection is limited toapproximately 0.3 mm, as the periphery of the actuator 136 comes intocontact with the internal surface 141. This maximum deflection islimited to protect the piezoelectric element 114 from excessive bending,and may be any suitable distance, for example between 0.2 and 0.5 mm. Itwill be appreciated that the components of the PCB and potting compoundform part of an actuating mass for excitation of the piezoelectricelement, with the actuator 136.

The housing 112 is injection moulded from plastics and is adapted towithstand the harsh environment within a pneumatic vehicle tyre. Thepiezoceramic disc 114, and actuator 136 and control circuitry form arethus part of a power generator for use in a preferred embodiment of theinvention.

The unit 100 operates substantially in the same way as the unit 10, asdescribed above therefore operation of the unit 100 is not described insignificant detail.

In summary, it will be appreciated that the units 10, 100 each serve asa telemetry unit, which is capable of measuring and transmitting datarelevant to tyre conditions local to the unit.

The concept of mounting an in-tyre telemetry unit to the inner surfaceof a tyre by means of a sacrificial footing 151 which can be permanentlybonded to the tyre is not limited to the application with units having apiezoelectric power generator as described above. The footing can beused with any suitable telemetry unit. Accordingly, the applicant mayclaim independent patent protection to this concept.

1. A telemetry unit for a tire monitoring apparatus, the telemetry unitincluding a housing having sensor means for measuring data relevant tothe local environment of the telemetry unit, transmission means fortransmitting measured data to a remote location, a piezoelectric powergenerator adapted to generate and store an electrical charge in responseto rotation of the tire for powering the unit, and control means forcontrolling the sensor means and transmission means, in which thecontrol means is arranged to vary the rate of transmission of data fromthe telemetry unit in dependence on the rotary speed of the tire, andwherein the piezoelectric power generator is arranged to produce atleast one pulse of electric charge for each rotation of the tire, andthe control means is adapted to monitor the number and/or frequency ofthe pulses generated in order to determine the appropriate rate oftransmission of data.
 2. A telemetry unit as claimed in claim 1, whereinthe control means is adapted to initiate monitoring of the localenvironment and transmission of measured data after a predeterminednumber of pulses has been detected.
 3. A telemetry unit as claimed inclaim 1, in which the housing is adapted to be mounted to an inner wallof a tire.
 4. A telemetry unit as claimed in claim 1, in which anexterior surface of the housing has a substantially arcuate profileadapted for bonding to an arcuate interior surface of a vehicle tire. 5.A telemetry unit as claimed in claim 1, in which an exterior surface ofthe housing includes an external profile for complimentary engagementwith the internal pattern of a vehicle tire.
 6. A telemetry unit asclaimed in claim 1, in which the housing is releasably mounted on afooting adapted to be bonded to the internal wall of a tire.
 7. Atelemetry unit as claimed in claim 6, in which the housing is releasablymounted on the footing by means of clips.
 8. A telemetry unit as claimedin claim 6, in which the footing includes air channels for allowingmovement of air about the housing, in use.
 9. A telemetry unit asclaimed in claim 1, in which the sensor means includes at least one of apressure sensor and/or a temperature sensor.
 10. A telemetry unit asclaimed in claim 1, in which an actuator is movable mounted in thehousing and adapted for contact with the piezoelectric element undercentrifugal forces.